New method to gauge onset and end of Indian monsoon

Agency | October 29, 2017 03:46 PM

BENGALURU: Predicting the behaviour of the Indian monsoon -- with all its complexities and factors that influence it -- has always been an annual gamble. For generations, scientists have struggled to produce a model for reliably predicting the duration and other parameters of the monsoon on a fine scale. Now, a team led by an Indian-origin scientist at the Florida State University (FSU) in the US has created a new methodology "for objectively defining the onset and demise of the Indian summer monsoon" at any location in the country.

Their study -- describing the methodology developed and tested by Associate Professor Vasubandhu Misra and co-workers at FSU -- is published in the journal Climate Dynamics.

The lack of a clear benchmark for the beginning and end of the monsoon for all areas of the country has been a longtime source of frustration and confusion for the general public, especially India's farmers. That will now end, the FSU scientists believe.

"We provide an objective definition of onset and demise of the Indian summer monsoon," Misra told this correspondent in an email. "The chance for detection of false onset and demise is minimised."

Current weather forecasting and monitoring protocols focus attention on the monsoon's onset in Kerala in the southwest corner of the country and extrapolating its progress for the rest of India.

Regional meteorological departments have relied on their own ad hoc criteria for determining the monsoon's onset, often leading to contradicting claims.

But in their report, Misra and co-workers argue that predicting the onset of monsoon for the entire country based on the date of arrival of monsoon rains in Kerala is "inadequate".

"Onset in the southwest corner of India is only somewhat tied to onset across the rest of India. In fact, it can take anywhere from days to weeks for the monsoon to reach other parts of India," the report says.

The FSU researchers claim their method for predicting the onset and demise of the Indian monsoon for different locations "is based solely on analysis of rain gauge measurements at these locations available from the India Meteorological Department".

They say the proposed definition is thus "unique in that it uses only the daily local rainfall information, thus simplifying the process of the diagnosis of the evolution of the monsoon" and effectively removing the necessity for extrapolation.

"We've tested this for 105 years of available data rain gauge observations, and this criterion hasn't failed once for any location over India," the authors claim.

"Our definition of onset and demise of the monsoon is solely based on rainfall and therefore it is comparatively easy to implement as it does not require multiple sources of data," they say. Forecasts made using this methodology will be "far more useful for practical applications".

"This is a nice piece of work since it provides high resolution information on onset date over every station," J. Srinivasan, distinguished scientist of the Divecha Centre for Climate Change at the Indian Institute of Science in Bengaluru, told this correspondent.

"The full impact of this new information is not easy to predict at this point," he added.

The FSU study was partly financed by the Earth System Science Organization of India's Ministry of Earth Sciences under Monsoon Mission.